Rhythmic contractions of a healthy heart are normally controlled by the sinoatrial (SA) node, specialized cells located in the upper right atrium. The SA node is the normal pacemaker of the heart, and when functioning normally, the heart produces rhythmic contractions and is capable of pumping blood throughout the body. However, due to disease or injury, the heart rhythm may become irregular resulting in diminished blood circulation. Arrhythmia is a general term used to describe heart rhythm irregularities arising from a variety of physical conditions and disease processes. Cardiac rhythm management systems, such as implantable pacemakers and implantable cardioverter/defibrillators (ICDs), have been used as an effective treatment for patients with serious arrhythmias. These systems typically comprise circuitry to sense electrical signals from the heart and a pulse generator for delivering electrical stimulation pulses to the heart. Leads extending into the patient's heart are connected to electrodes electrically coupled to the myocardium for sensing the heart's electrical signals and for delivering stimulation pulses to the heart in accordance with various therapies for treating the arrhythmias.
Cardiac rhythm management systems operate to stimulate the heart tissue adjacent to the electrodes to produce contractions of the tissue. Pacemakers are cardiac rhythm management systems that deliver a series of low energy pace pulses timed to assist the heart in producing a contractile rhythm that maintains cardiac pumping efficiency. Pace pulses may be intermittent or continuous, depending on the needs of the patient. There exist a number of categories of pacemaker devices, with various modes for sensing and pacing one or more heart chambers.
When a pace pulse produces a contractile response in heart tissue, the contractile response is typically referred to as capture, and the electrical cardiac signal corresponding to capture is denoted the evoked response. Superimposed on the evoked response may be a pacing artifact signal that includes a signal associated with post pace residual polarization. The magnitude of the pacing artifact signal may be affected by a variety of factors including lead polarization, after potential from the pace pulse, lead impedance, patient impedance, pace pulse width, and pace pulse amplitude, for example.
A pace pulse must exceed a minimum energy value, or capture threshold, to produce a contraction. It is desirable for a pace pulse to have sufficient energy to stimulate capture of the heart without expending energy significantly in excess of the capture threshold. If the pace pulse energy is too low, the pace pulses may not reliably produce a contractile response in the heart resulting in ineffective pacing. If the pace pulse energy is too high, the result may be patient discomfort as well as shorter battery life. Thus, accurate determination of the capture threshold is required for efficient pace energy management.
Capture verification has been accomplished by examining the cardiac waveform following a pacing pulse. Such an examination may involve, for example, comparison of the cardiac waveform following the pace pulse to a morphology template characterizing a captured response. If the cardiac waveform matches the captured response template, then capture may be declared.
Morphology templates may also be used to detect and/or classify types of cardiac tachyarrhythmias. Implantable cardioverter/defibrillators (ICDs) have been used to deliver effective treatment for patients with serious tachyarrhythmias. ICDs are able to treat such arrhythmias with a variety of tiered therapies. These tiered therapies include providing anti-tachycardia pacing or cardioversion energy for treating ventricular tachycardia, and/or delivering defibrillation energy to the heart for treating ventricular fibrillation. To effectively deliver an appropriate therapy, the ICD may first identify the type of arrhythmia that is occurring. To identify the type of arrhythmia, the ICD may compare sensed cardiac signals to a previously stored morphology template.
Detecting and classifying various types of cardiac events using morphology templates depends in part upon how accurately the morphology template represents the type of cardiac event. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading the present specification, there is a need in the art for methods and systems that reliably and accurately characterize cardiac waveforms representing various cardiac events. There exists a further need for such an approach that is adaptive and accommodates to changes in cardiac signal morphology over time. The present invention fulfills these and other needs.